In semiconductor manufacturing processes such as, for example, the sputtering of thin films onto substrates such as those formed of silicon it is necessary to hold substrate wafers in place for processing. While held, a sequence of processes is performed on the wafer, many of which result in the application of a thin film or coating layer to the wafer. Wafers held for the application of such coatings may be held in a horizontal orientation, facing upwardly or downwardly, or may be held in a vertical orientation facing in a horizontal direction. In all such coating processes, the wafer to be coated must be retained securely to a holder in a generally stress free state. To hold the wafers in such a state and to move the wafers safely and quickly between various processors and positions, wafer holders or clamps that employ resiliently supported continuous clamping rings to urge the wafer uniformly around its edge against the wafer holder have gained wide acceptance.
Wafer holders that employ wafer clamping rings are exposed to a range of temperatures when holding a wafer in a vacuum chamber for processing. These rings are further exposed to deposition of the same materials that are being deposited onto the wafers. While each wafer is subjected to a single cycle in a processing chamber in which the temperatures may rise and fall, and where a film thickness measured in microns is deposited on the wafer, the holders, and particularly the clamping rings that urge the wafers against the holders, are exposed to a large number of cycles in each of which the temperatures are cycled from maximum to minimum and in the course of which multiple layers of coating material accumulate on the clamping ring surfaces.
Over the course of many cycles, the clamping rings are observed to warp. The warping of a clamping ring is typically a permanent deformation of the clamping ring that alters the way in which the ring contacts the wafer. When the warping becomes excessive, the ring no longer adequately clamps the wafer. As a result, wafers can move in the holder, can be unevenly subjected to clamping forces in the holder causing breakage, or are not held in the proper position. The amount of warping and the number of cycles that it takes to cause excessive warping is observed to vary with different coating materials and coating processes. When the warping becomes excessive, after a number of wafers have been processed, it is necessary to replace the clamping ring with a new ring having its original design shape that will clamp the wafer uniformly around its edge.
For example, in one processing sequence in which the backside of a semiconductor is coated with two deposition layers of tantalum (Ta) and one layer of gold (Au), it is found that after only several hundred wafers are processed, excessive warping of the inner diameter of the clamping ring occurs lifting the inner rim out of its normal plane and toward the deposition chamber. As a result, a chamber overhaul to replace the clamping ring is required after far fewer wafers are processed than the several thousand desired.
Replacement of the clamping ring is necessary to prevent damage to the wafers and results in a loss of expensive production time in the making of semiconductors. The warping problem has not been effectively solved.
Accordingly, there remains a need to prevent or substantially delay the onset of excessive clamp ring warping in semiconductor wafer holders used in film deposition processes.